Vehicle vapor storage canisters must have a bed of adsorbent material of a volume sufficient to adsorb the greatest expected volume of fuel vapor emissions. Vehicle fuel tanks normally generate fuel vapors internally more or less continually when they are closed. This daily, per drive cycle volume of fuel vapors is generally referred to as the diurnal vapors or diurnal emissions, and are routed to the storage canister to avoid venting them to atmosphere. Purge air flow created b a source of purge vacuum, generally manifold vacuum, is then applied to the canister to pull outside air through the canister, drawing the stored vapors into the engine for combustion.
The efficiency of the purge operation may be quantified in terms of a dimensionless efficiency factor defined as the volume of purge air flow that is pulled through the adsorbent bed divided by the volume of the adsorbent bed. For a given volume of adsorbent, more purge air flow gives a more thorough purge. For a given purge air flow, a lesser volume of adsorbent is more thoroughly purged. The purge air flow available per drive cycle is limited, but the volume of diurnal vapors is generally small enough that the volume of the canister may in turn be small enough to give a high efficiency factor.
A new problem presents itself, however, when it is desired to also store and recover those fuel vapors that are displaced from the tank when it is filled, generally called the fuel fill vapors. Although these are generated less frequently, perhaps every ten drive cycles or so, they have a greater volume than the diurnal vapors, especially with large tanks that are mostly empty when filled. Consequently, the adsorbent bed volume must be greater. If that greater adsorbent volume were loaded with just the diurnal vapors, and then purged with the same, limited flow of purge air per drive cycle, the efficiency factor would consequently be lower. For example, all things being equal, the efficiency factor would be one third as great if the total adsorbent bed volume were tripled.